Solemn declarations are often used in official and serious settings, and there are many synonyms that can be used for this phrase. Some alternatives include solemn oaths, formal promises, sworn affirmations, and serious avowals. Other options might include grave pronouncements, weighty proclamations, formal attestations, or significant pledges. Depending on the context and the tone of the situation, different synonyms might be more appropriate to use. However, all of these phrases convey a sense of gravity and importance, suggesting that the speaker is making a significant statement that carries weight and may have repercussions in the future.